1. Field of the Invention
The present invention relates to processes for the partial oxidation of a whole crude oil feedstock in a membrane wall gasification reactor to produce a synthesis gas and electricity.
2. Description of Related Art
Gasification is well known in the art and it is practiced worldwide with application to solids and heavy liquid fossil fuels, including refinery bottoms. The gasification process uses partial oxidation to convert carbonaceous materials, such as coal, petroleum, biofuel, or biomass with oxygen at high temperature, i.e., greater than 800° C., into synthesis gas (“syngas”), steam and electricity. The syngas consisting of carbon monoxide and hydrogen can be burned directly in internal combustion engines, or used in the manufacture of various chemicals, such as methanol via known synthesis processes and to make synthetic fuels via the Fischer-Tropsch process.
In refinery operations, the main process block is known as the Integrated Gasification Combined Cycle (IGCC), which converts the feedstock into hydrogen, power and steam. FIG. 1 shows the process flow diagram of a conventional IGCC of the prior art, which includes a feed preparation section 102, a gasification reactor 104, an air separation unit 180, a syngas quench and cooling unit 110, a water-gas shift reactor 120, an acid gas removal (AGR) and sulfur recovery unit (SRU) 130, a gas turbine 140, a heat recovery steam generator (HRSG) 150, and a steam turbine 160.
In a conventional IGCC, a feedstock is introduced via a feed line 101 to the feed preparation section 102. The prepared feedstock is then passed to the gasification reactor 104 with a predetermined amount of oxygen 103 produced from the air separation unit 180. The feedstock is partially oxidized in the gasification reactor 104 to produce a hot syngas 106 which is conveyed to the syngas quench and cooling unit 110. Hot syngas 106 is cooled with boiler feed water 156 to produce cooled syngas 114 and steam. A portion of the steam 112 is passed to and used in the water-gas shift reactor 120 to produce shifted gas 122, and the remaining portion of the steam 116 is consumed in the HRSG 150. Shifted gas 122 is treated in the AGR/SRU 130 to separate and discharge carbon dioxide 136, sulfur 138; a portion of the hydrogen syngas which is recovered at 132. A second portion of the hydrogen syngas, identified as gas turbine feed 134, is passed to the gas turbine 140 with air feed 142 and combusted to produce electricity 144. The high pressure combustion gas discharge 146 from the gas turbine 140 is conveyed to the HRSG 150 to generate steam which is used in the steam turbine 160 to produce additional electricity 162.
The air separation unit 180 and most of the downstream processes utilize mature technologies with high on-stream reliability factors. However, the gasification reactor 104 has a relatively limited lifetime which can be as short as from 3 to 18 months, depending upon the characteristics of the feed and the design of the reactor.
The three principal types of gasification reactor technologies are the moving bed, fluidized bed and entrained-flow systems. Each of the three types can be used with solid fuels, but only the entrained-flow reactor has been demonstrated to efficiently process liquid fuels. In an entrained-flow reactor, the fuel, oxygen and steam are injected at the top of the gasifier through a co-annular burner. The gasification usually takes place in a refractory-lined vessel which operates at a pressure of about 40 bars to 60 bars and a temperature in the range of from 1300° C. to 1700° C.
There are two types of gasifier wall construction: refractory and membrane. The gasifier conventionally uses refractory liners to protect the reactor vessel from corrosive slag, thermal cycling, and the elevated temperatures that range from 1400° C. to 1700° C. The refractory is subjected to the penetration of corrosive components from the generation of the syngas and slag and, thereafter, to subsequent reactions in which the reactants undergo significant volume changes that result in strength degradation of the refractory materials. The replacement of the degraded refractory linings can cost several millions of dollars a year and several weeks of downtime for a given reactor. Up until now, the solution has been the installation of a second or parallel gasifier to provide the necessary continuous operating capability during maintenance downtime, but the undesirable consequence of this duplication is a significant increase in the capital costs associated with the unit operation.
An alternative membrane wall gasifier technology uses a cooling screen protected by a layer of refractory material to provide a surface on which the molten slag solidifies and flows downwardly to the quench zone at the bottom of the reactor. The advantages of the membrane wall reactor include reduced reactor dimensions as compared to other systems; a significantly greater average on-stream time of 90%, as compared to an on-stream time of 50% for a refractory wall reactor; elimination of the need to have a parallel reactor to maintain continuous operation as in the case of refractory wall reactors; and the build-up of a layer of solid and liquid slag that provides self-protection to the water-cooled wall sections.
In a membrane wall gasifier, the build-up of a layer of solidified mineral ash slag on the wall acts as an additional protective surface and insulator to minimize or reduce refractory degradation and heat losses through the wall. The water-cooled reactor design also avoids what is termed “hot wall” gasifier operation, which requires the construction of thick multiple-layers of expensive refractories which are subject to degradation. In the membrane wall reactor, the slag layer is renewed continuously with the deposit of solids on the relatively cool surface. Further advantages include shorter start-up/shut down times; lower maintenance costs than the refractory type reactor; and the capability of gasifying feedstocks with high ash content, thereby providing greater flexibility in treating a wider range of coals, petcoke, coal/petcoke blends, biomass co-feed and liquid feedstocks.
There are two principal types of membrane wall reactor designs that are adapted to process solid feedstocks. One such reactor uses vertical tubes in an up-flow process equipped with several burners for solid fuels, e.g., petcoke. A second solid feedstock reactor uses spiral tubes and down-flow processing for all fuels. For solid fuels, a single burner having a thermal output of about 500 MWt has been developed for commercial use. In both of these reactors, the flow of pressurized cooling water in the tubes is controlled to cool the refractory and ensure the downward flow of the molten slag. Both systems have demonstrated high utility with solid fuels, but not with liquid fuels.
The gasification reactor is operated to produce synthesis gas, or syngas. For the production of liquid fuels and petrochemicals, the key parameter is the mole ratio of hydrogen-to-carbon monoxide in the dry syngas. This ratio is usually between 0.85:1 and 1.2:1, depending upon the characteristics of the feedstock. Additional treatment of the syngas is needed to increase this ratio of hydrocarbon-to-carbon to 2:1 for Fischer-Tropsch applications, or to produce additional hydrogen through the water-gas shift reaction represented by CO+H2O→CO2+H2. In some cases, part of the syngas is burned together with some of the off gases in a combined cycle to produce electricity and steam. The overall efficiency of this process is between 44% and 48%.
While the gasification process is well developed and suitable for its intended purposes, its applications in conjunction with whole crude oil processes have been limited. In a typical refinery, whole crude oil is initially processed in an atmospheric distillation column or a crude tower where it is separated into a variety of different components including naphtha boiling in the range of from 36° C. to 180° C., diesel boiling in the range of from 180° C. to 370° C., and atmospheric bottoms boiling above 370° C. The atmospheric bottoms residue is further processed in a vacuum distillation column where it is separated into a vacuum gas oil (VGO) boiling in the range of from 370° C. to 520° C. and a heavy vacuum residue boiling above 520° C. The VGO can be further processed by hydrocracking to produce naphtha and diesel, or by fluid catalytic cracking (FCC) to produce gasoline and cycle oils. The heavy vacuum residue can be treated to remove unwanted impurities or converted into useful hydrocarbon products.
The problem addressed by the present invention is that of directly converting a whole crude oil feedstock of relatively low value in a process that is economically viable, and that is capable of producing a syngas and/or an enriched hydrogen system that can be used as a feedstream for other processes in the same refinery, or used to produce methanol and/or synthetic fuels.